Recent automatic transmissions often comply a clutch-to-clutch shifting system, i.e., an independent shifting system in which hydraulic control for an off-going clutch is not performed during an upshift or a downshift, and only release timing of the off-going clutch is controlled. Therefore, for the off-going clutch, after the release timing, the hydraulic pressure is released from the off-going friction element without additional control.
In such a hydraulic control scheme, the timing of release of the hydraulic pressure from the off-going friction element is determined as a point at which a hydraulic pressure for an on-coming friction element rises suitably. When the hydraulic pressure is released too quickly from the off-going friction element or when the hydraulic pressure is supplied too slowly to the on-coming friction element, a flare may occur such that shift quality may deteriorate.
Although an additional shift control scheme (e.g., a bang-bang control) can be used in order to solve such a problem, the shift feel may be deteriorated by torque changes due to the rapid increase of hydraulic pressure.
In particular, during a power-on 2-3 upshift, a flare phenomenon, in which turbine rpm abnormally increases according to hydraulic pressure states, may occur. Such phenomenon of a rapid increase of turbine rpm is caused by a release of hydraulic pressure from an off-going clutch in a state in which hydraulic pressure of an on-coming clutch does not sufficiently sustain an input torque (i.e., turbine torque).
The information disclosed in this Background of the Invention section is only for enhancement of understanding of the background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art that is already known to a person skilled in the art.